The effect of reducing the length of the lighting period in late pregnancy on milk production and related hormones in native black goats

Seasonal breeding mammals are sensitive to changes in day length. Thus, in small ruminants from temperate and subtropical latitudes, photoperiod is the main environmental factor that modulates their pattern of reproductive activity (Duarte et al., 2010). However, natural oscillations in the photoperiod or alterations by artificial photoperiod can lead to changes in milk production in ruminants with seasonal and non seasonal reproduction (Dahl et al., 2000). Exposure to artificially shortened photoperiods during the dry period increased their milk yields in the subsequent lactation (Auchtung et al., 2005; Mabjeesh et al., 2007). In the 12 wk of subsequent lactation, greater milk yields have been reported for does that received artificial short days during the dry period than those that received long days (Mabjeesh et al., 2007). Many livestock breeders in cold regions keep their animals in closed places in late autumn and during winter. In the closed position, the amount and intensity of light can be controlled. Short day length at the end of the pregnancy period is associated with increased milk production in lactating animals, but there is no information in this field for native goats in the country. Therefore, the aim of this research was to investigate the effect of reducing the length of the lighting period in late pregnancy on milk production and related hormones in black goats native to Chaharmahal and Bakhtiari province.

The work was conducted in the Farsan city (50º and 35' minutes of longitude and 32º and 16' of latitude and 2020 meters above sea level) located in Chaharmahal and Bakhtiari provinces. 40 indigenous black goats were randomly assigned to one of the following two treatments. Treatment 1: control; Female goats that were exposed to natural day length during pregnancy (20 heads). Treatment 2: Female goats that were exposed to short day length (8 hours of light and 16 hours of darkness) during the last 45 days of pregnancy (20 head). The female goats of two different treatments were reared in two separate locations of the same size with the same environmental and management conditions during the late pregnancy period (experimental period). The quantity and quality of feed was similar for both groups. After giving birth, all female goats were exposed to natural day length. Hand milking was done at one week intervals from one week to one month after birth. Blood samples were taken from jugular vein at the beginning of the experiment, 20 days before parturition, at the time of parturition, 15 and 30 days after parturition.

In the first week, milk production in the experimental treatment was 11% more than the control treatment, but in the second, third and fourth weeks, this superiority reached 13, 16 and 21%, respectively. On average, milk production in the experimental treatment was 15% more than the control treatment. In dairy goats, it was reported that milk production in the subsequent lactation period was 26% higher in goats that were exposed to short day length conditions in the last third of their pregnancy than goats that were exposed to long day conditions at the same time (Mabjeesh et al., 2007).The interaction between the treatment and the milk measurement time was not statistically significant. Milk production peaked in the third week in the control group and in the fourth week in the experimental treatment. Changing the duration of lighting at the end of pregnancy did not have a significant effect on any of the milk compounds on the seventh, fourteenth, twenty-first and twenty-eighth days of lactation. Normally, the composition of milk is not affected by photoperiod (Mabjeesh et al., 2007).The amount of prolactin hormone in the control treatment 20 days before delivery was higher than the amount of prolactin hormone in the experimental treatment at the same time (p<0.05). It has been found that short day length reduces the secretion of prolactin (Smith 1998). In both treatments, the amount of prolactin hormone increased from 45 days before delivery to 30 days after delivery. At the time of parturition, the amount of prolactin hormone in female goats of both treatments was similar. Within each treatment, the amount of prolactin hormone in female goats 15 days after giving birth was higher than its value at the time of giving birth, and the amount of prolactin hormone at the time of giving birth was higher than its value at 20 days before giving birth, and these differences were statistically significant (p<0.01). However, within each treatment, the difference in the amount of prolactin hormone in female goats between the 30th and 15th days after birth was not significant. The interaction effect of treatment and prolactin measurement time was statistically significant (p<0.05). Within each treatment, the amount of prolactin hormone in female goats 15 days after giving birth was higher than its amount at the time of giving birth, and the amount of prolactin hormone at the time of giving birth was higher than its value at 20 days before giving birth, and this difference were statistically significant (p<0.01). However, within each treatment, the difference in the amount of prolactin hormone in female goats between the 30th and 15th days after birth was not significant. The interaction effect of treatment and prolactin measurement time was statistically significant (p<0.05). The average melatonin hormone before and during delivery in treatment 2 was higher than the control treatment (p<0.01), but after delivery the difference between the two treatments was not significant. The amount of IGF 1 hormone in female goats of both treatments decreased from 45 days before calving to the time of calving and then increased.The amount of IGF-1 hormone in the experimental treatment was higher than the control treatment in 20 days before delivery (p<0.01) and at the time of delivery (p<0.05).

As a result, reducing the length of the lighting period at the end of pregnancy increases the milk production of native black goats in the subsequent lactation period.